In the context of seven busy obstetrical services we are studying factors responsible for variability in human placental mixed function oxidase and glutathione S-transferase activities. Specific attention is being directed toward documenting environmental factors which influence placental microsomal activity toward benzo(a)pyrene and 7-ethoxyresorufin and placental cytosolic gluthathione S-transferase activity toward 1-chloro 2,4-dinitrobenzene. Active maternal cigarette smoking as well as passive exposure to cigarette smoke are being currently investigated. In addition, genetic aspects of variability are under investigation in dichorionic twin placentas. Material from both normal and abnormal pregnancies is included.